This invention is directed to a fill valve which is responsive to liquid level and is particularly suitable for stopping the filling of liquid propane tank and other liquid petroleum gas tanks when the liquid level therein reaches a predetermined fill point to prevent tank overfilling.
Liquefied petroleum gas, including propane, butane and mixtures thereof with other gases of similar nature, is extensively used for household cooking and heating where natural gas is unavailable. Furthermore, liquefied petroleum gas is used for these purposes on recreational vehicles. Liquefied petroleum gas is extremely volatile and burns readily. In the liquefied state, the vapor pressure of propane and butane is such that the liquid can be stored in tanks of only moderate strength in outdoor locations at the ambient temperatures encountered. These characteristics make liquefied petroleum gas extremely desirable for use as fuels.
Liquid petroleum gases have extremely high thermal coefficients of expansion, such that even moderate changes in temperature thereof may cause the liquid to expand or contract appreciably. For this reason, it is necessary to maintain a head space in a tank where the liquefied petroleum gas is stored. A portion of the volume of any such tank must be left unoccupied with liquid to allow for expansion upon increase in temperature.
The most common present device for determining maximum tank fill comprises a liquid-sensing dip tube which extends down into the tank to the point of highest proper liquid level in the tank. The dip tube vents outside the tank and has a manual valve thereon. The manual valve is opened, and filling is commenced. Vapor from the tank head space is discharged out of the open manual valve and, when the tank is filled with liquid to the bottom of the dip tube, the liquid level is sensed thereat by the liquid discharging from the manual vent valve. The liquid expands and vaporizes as it reaches the atmosphere, thus creating a telltale white cloud. When seeing such a cloud, the operator shuts off the fill valve and terminates the filling operation.
To reduce reliance upon the operator, it is desirable that an automatic fill-stop valve is required. The present inventor, Gerald D. Mylander, has previously invented the subject matter of U.S. Pat. No. 3,363,641 which is responsive to the liquid level so that tank filling is stopped when the desired level is reached. That structure is arranged so that both the filling flow and its level-sensing flow are automatically shut off. In addition, he has invented an automatic fill-stop valve, as shown in U.S. Pat. No. 4,191,208 which is of simpler construction, but has features therein which increase the reliability of shifting of the main valve spool for shutoff of in-flowing liquid.
The vapor pressure over the liquid petroleum gas and its vapor in a tank at outdoor temperatures above freezing is adequate to reliably shift the main shutoff valve spool. However, when the temperature is low, for example in the order of 20 degrees F., the vapor pressure is too low for reliable operation. Under low temperature conditions, the automatic fill-stop valve shown in U.S. Pat. 4,191,208 does shift, but the reliability can be enhanced by providing a pressure booster which momentarily provides an increase in pressure in the volume which acts upon the main shutoff spool to urge it in the closing direction.